[Kotlin][Client] Added supporting Retrofit2(RxJava/RxJava2/Coroutines) (#5750)

* Added support Retrofit2(RxJava/RxJava2/Coroutines) to Kotlin client code generator

* Added generated samples for Retrofit2(RxJava/RxJava2/Coroutines) on Kotlin

* Fixed generating retrofit2 without Rx/Coroutines

* Fixed MultipartBody template, remove redundant space after MultipartBody.Part annotation

* Fix documentation diff.

* Fix generating build.gradle file for samples.
Add dependencies for rxJava/rxJava2 and rx retrofit's adapter.

* Update generated sample with rx dependencies

* Update generated sample with coroutines

* Update generated sample with RxJava 2 dependencies

* Update and refactoring scripts for sample generation

* Update generated sample code

* revert changes by mistake

* revert changes by mistake #2

* Fix return type for first RxJava

* Add RxJavaCallAdapterFactory to ApiClient scheme for RxJava 1/2

* Fix script loggin

* Update generated code for RxJava 1/2

* Fix kotlin.md documentation by script export_docs_generators.sh

* Update Kotlin samples project in pom.xml

* Revert "Update Kotlin samples project in pom.xml"

This reverts commit 9de4d0ba

* Fixed "" wrapping number types for annotation value
Fixed generating polymorphic interfaces for Gson, interface fields can't be marked with @SerializedName annotation, it's enough to mark it in child model.
Fixed instantiationTypes(array, list, map) for Kotlin generator

* Update Kotlin samples project after last fixes

* Update Kotlin samples project for kotlin-jvm-*

* Update Kotlin readme.md documentations

* Update Kotlin client sample changes

* Fixed encoding braces for generics model

* Update Kotlin client sample changes

* Update Kotlin client sample after merge with master

* Fixed adding empty braces for inherit from Map/Array

* Update sample model after last fix with empty braces

* Revert adding @SerializedName to Kotlin interface fields through @get:SerializedName as for Jackson

* Update Kotlin client samples with adding @get:SerializedName

samples/openapi3/client/petstore/kotlin-jvm-retrofit2-rx2/docs/AnotherFakeApi.md

@@ -0,0 +1,56 @@
+# AnotherFakeApi
+
+All URIs are relative to *http://petstore.swagger.io:80/v2*
+
+Method | HTTP request | Description
+------------- | ------------- | -------------
+[**call123testSpecialTags**](AnotherFakeApi.md#call123testSpecialTags) | **PATCH** /another-fake/dummy | To test special tags
+
+
+<a name="call123testSpecialTags"></a>
+# **call123testSpecialTags**
+> Client call123testSpecialTags(client)
+
+To test special tags
+
+To test special tags and operation ID starting with number
+
+### Example
+```kotlin
+// Import classes:
+//import org.openapitools.client.infrastructure.*
+//import org.openapitools.client.models.*
+
+val apiInstance = AnotherFakeApi()
+val client : Client =  // Client | client model
+try {
+    val result : Client = apiInstance.call123testSpecialTags(client)
+    println(result)
+} catch (e: ClientException) {
+    println("4xx response calling AnotherFakeApi#call123testSpecialTags")
+    e.printStackTrace()
+} catch (e: ServerException) {
+    println("5xx response calling AnotherFakeApi#call123testSpecialTags")
+    e.printStackTrace()
+}
+```
+
+### Parameters
+
+Name | Type | Description  | Notes
+------------- | ------------- | ------------- | -------------
+ **client** | [**Client**](Client.md)| client model |
+
+### Return type
+
+[**Client**](Client.md)
+
+### Authorization
+
+No authorization required
+
+### HTTP request headers
+
+ - **Content-Type**: application/json
+ - **Accept**: application/json
+